Try and reuse probcli when loading or re-loading a model

Try to reuse the existing probcli when switching to a new machine or at least when reloading a machine.

This should improve reloading performance: re-loading a simple machine takes about 3 seconds and a machine like the AMASS scheduler takes 5 seconds.
This is not a real problem, but if a simple fix can improve this performance, that would be nice.
For comparison, the former is instantaneous in ProB Tcl/Tk and the latter takes about 1 second.
(NoteroB Tcl/Tk always uses the same probcli for animation.)
I think most of this time in ProB2 UI is lost due to starting up a new probcli?